5. ADDITIONAL ASPECTS

Cosmological simulations not only have been used to investigate the
large-scale distribution and physical state of the warm-hot
intergalactic medium, they also have
been applied to predict statistical properties of high-ion absorption
systems that can be readily compared with the UV and X-ray measurements
(e.g.,
Cen et al. 2001;
Fang & Bryan
2001;
Chen et al.2003;
Furlanetto et
al. 2005;
Tumlinson
& Fang 2005;
Cen & Fang
2006).
Usually, a large number of artificial spectra along random sight-lines
through the simulated volume are generated. Sometimes, instrumental
properties of existing spectrographs and noise characteristics are
modelled, too (e.g.,
Fangano et
al. 2007).
The most important quantities derived from
artificial spectra that can be compared with observational data are the
cumulative and differential number densities (dN /
dz) of O VI,
O VII, O VIII systems as a function of the absorption equivalent
width. An example for this is shown in Fig. 8.
Generally, there is a good
match between the simulations and observations for the overall shape of
the dN / dz distribution (see also
Sect. 3.2), but mild discrepancies exist
at either low or high equivalent widths, depending on what simulation is
used (see, e.g.,
Tripp et
al. 2007).
For the interpretation of such discrepancies
it is important to keep in mind that the different simulations are based
on different physical models for the gas, e.g., some simulations
include galaxy feedback models, galactic wind models, non-equilibrium
ionisation conditions, etc., others do not. For more information on
numerical simulations of the WHIM see
Bertone et
al. 2008
- Chapter 14, this volume.

Figure 8. The differential number of
intervening oxygen high-ion
(O VI, O VII,O VIII) absorbers in the WHIM in a cosmological simulation is
plotted against the equivalent width of the absorption
(for details see
Cen & Fang
2006).
While for O VII and O VIII no significant observational results are
available to be compared with the simulated spectra (see
Sect. 4.2),
the predicted frequency of O VI absorbers is in good agreement with the
observations (Sect. 3.2). Adapted from
Cen & Fang
(2006).

WHIM simulations also have been used to investigate the frequency and
nature of BLAs at low redshift
(Richter et
al. 2006b).
As the simulations suggest, BLAs indeed host
a substantial fraction of the baryons at z = 0. From the
artificial UV spectra generated from their simulation Richter et
al. derive a number of BLAs per unit redshift
of (dN / dz)BLA 38 for H I absorbers
with log (N(cm-2) / b(km s-1))
10.7, b 40 km s-1, and
total hydrogen column densities N(H II)
1020.5
cm-2. The baryon content of these systems is ~ 25 percent
of the total baryon budget in the
simulation. These results are roughly in line with the observations if
partial photoionisation of BLAs is taken into account
(Richter et
al. 2006a;
Lehner et
al. 2007).
From the simulation further follows that BLAs predominantly trace
shock-heated collisionally ionised WHIM gas at temperatures log
T 4.4-6.2. Yet,
about 30 percent of the BLAs in the simulation originate in the
photoionised Ly forest
(log T < 4.3) and their large line widths are
determined by non-thermal broadening effects such as unresolved velocity
structure and macroscopic turbulence. Fig. 9
shows two examples of the velocity profiles of BLAs generated from
simulations presented in
Richter et
al. (2006b).

Figure 9. Two examples for BLA absorbers
from the WHIM in a cosmological simulation are shown. The panels show
the logarithmic total hydrogen volume density, gas temperature, neutral
hydrogen volume density, and normalised intensity for H I
Ly and O
VI 1031.9
absorption as a function of the radial velocity along each
sightline. From
Richter et
al. (2006b).

The results from the analysis of artificially generated UV spectra
underline that the comparison between WHIM simulations and quasar
absorption line studies indeed are
quite important for improving both the physical models in cosmological
simulations and the strategies for future observations of the warm-hot
intergalactic gas.

Although this chapter concentrates on the properties of WHIM absorbers
at low redshift (as visible in UV and X-ray absorption) a few words
about high-ion absorption at
high redshifts (z > 2) shall be given at this point. At
redshifts z > 2, by far most of the baryons are residing in
the photoionised intergalactic medium that gives
rise to the Ly
forest. At this early epoch of the Universe, baryons situated in
galaxies and in warm-hot intergalactic gas created by large-scale
structure formation contribute together with only < 15 percent to the
total baryon content of the Universe. Despite the relative unimportant
role of the WHIM at high z, O VI absorbers are commonly found in
optical spectra of high-redshift quasars (e.g.,
Bergeron et
al. 2002;
Carswell et
al. 2002;
Simcoe et
al. 2004).
The observation of intervening O VI absorbers at high redshift is much
easier than in the local Universe, since the absorption
features are redshifted into the optical regime and thus are easily
accessible with ground-based observatories. However, blending problems
with the numerous H I
Ly forest lines at high
z are much more severe than for low-redshift sightlines. Because
of the higher intensity of the metagalactic UV background at high
redshift it is expected that many of the O VI systems in the early
Universe are photoionised. Collisional ionisation of O VI yet may be
important for high-redshift absorbers that originate in galactic winds
(see, e.g.,
Fangano et
al. 2007).
While for low redshifts the population of O VI
absorbers is important for the search of the "mission baryons" that are
locked in the WHIM phase in the local Universe, O VI absorbers at high
redshift are believed to represent a solution for the problem of the
"missing metals" in the early epochs of structure formation. This
problem arises from the facts that at high
redshift an IGM metallicity of ~ 0.04 is expected from the
star-formation activity of Lyman-Break Galaxies (LBGs), while
observations of intervening C IV
absorption systems suggest an IGM abundance of only ~ 0.001 solar
(Songaila 2001;
Scannapieco
et al. 2006),
thus more than one order of magnitude too low. Possibly, most of the
missing metals at high z are hidden in highly-ionised hot gaseous
halos that surround the star-forming galaxies (e.g.,
Ferrara et
al. 2005)
and thus should be detectable only with high ions such as O VI rather
than with intermediate ions such as C IV.
Using the UVES spectrograph installed on the Very Large Telescope
(VLT)
Bergeron &
Herbert-Fort (2005)
have studied the properties of high-redshift
O VI absorbers along ten QSO sightlines and have found possible evidence
for such a scenario. Additional studies are required to investigate the
nature of high-z O VI systems and their relation to galactic
structures in more detail. However, from the existing measurements
clearly follows that the study of
high-ion absorbers at large redshifts is of great importance to our
understanding of the formation and evolution of galactic structures at
high z and the transport of metals into the IGM.

The analysis of absorption features from high ions of heavy elements and
neutral hydrogen currently represents the best method to study baryon
content, physical properties, and distribution of the warm-hot
intergalactic gas in large-scale filaments at low and high
redshift. However, the interpretation of these spectral signatures in
terms of WHIM baryon content and origin still is afflicted with rather
large systematic uncertainties due to the limited data quality and the
often poorly known physical conditions in WHIM absorbers (e.g.,
ionisation conditions, metal content, etc.). Future instruments in the
UV (e.g., COS) and in the X-ray band (e.g., XEUS,
Constellation X) hold the prospect of providing large amounts of
new data on the WHIM with good signal-to-noise ratios and substantially
improved absorber statistics. These missions therefore will be of great
importance to improve our understanding of this important intergalactic
gas phase.

Acknowledgements.
The authors thank ISSI (Bern) for support of the team "Non-virialized
X-ray components in clusters of galaxies". SRON is supported fiancially
by NWO, the Netherlands Organization for Scientific Research.